Secretary of State John Kerry has signaled that the US hopes to enlist the foreign-backed Syria insurgents that the Obama administration has decided to arm and train to battle against both the Syrian government and the "extremists" in neighboring Iraq.

"Obviously, in light of what has happened in Iraq, we have even more to talk about in terms of the moderate opposition in Syria, which has the ability to be a very important player in pushing back against the so-called Islamic State of Iraq and Levant (ISIL)'s presence and to have them not just in Syria, but also in Iraq," Kerry said at the start of a meeting with the so-called Free Syrian Army leader Ahmad al-Jarba on Friday.

A senior State Department official traveling with Kerry later claimed the secretary did not mean to imply that Syria-based insurgents would actually cross the border to fight in Iraq. The official was not authorized to brief reporters by name and spoke on condition of anonymity.

Obama sent Congress a $500 million request Thursday for a Pentagon-run program that would significantly expand previous covert efforts to arm foreign-sponsored insurgents fighting both the extremists and Syrian army forces.

If approved by lawmakers, the program would in effect open a second front in the fight against ISIL that is spilling over Syria's border and threatening to destabilize Iraq.

Al-Jarba thanked the Obama administration for requesting the $500 million, but said his militants want even more foreign aid to fight two fronts: a bloody insurgency and their unsuccessful effort in Syria.

"We still need greater assistance," al-Jarba said, speaking through a translator. "We hope for greater cooperation with the US" He said General Abdullah al-Bashir, the head of the military wing of the Syrian opposition, "is ready to cooperate with the US side."

Kerry traveled through the Mideast over the last days to try to broker a political agreement with Iraqi leaders to give more authority to opponents.

Kerry also met with King Abdullah of Saudi Arabia, where it was expected he would seek the monarch's help in supporting efforts to combat the Takfiri insurgency.